1. Field of the Invention
This invention relates to a plant and method for preheating, calcining, and sintering granular or pulverulent raw materials, such as cement raw meal.
2. Description of the Prior Art
In the technology pertaining to the heat treatment of granular or pulverulent raw materials such as cement raw meal, it is always desirable to utilize heat energy as much as possible. My, commonly assigned U.S. Pat. No. 3,864,075 dated Feb. 4, 1975, discloses a plant which is characterized by the division of the hot waste cooling air leaving the cooler so that a portion is directed as combustion air to the burner of the kiln, and a portion is directed to the air inlet of one heat exchanger unit. The gas inlet of another heat exchanger unit is connected to an exhaust gas outlet of the kiln and the heat exchanger units each have at or near its gas (or air) outlet end, means associated therewith for controlling the air or gas flow through that unit and hence the division of the heated cooling air.
In a preferred embodiment of my earlier development, one heat exchanger unit has an air inlet to which hot waste cooling air is directed, with means being provided for increasing the heat content of the air. The other heat exchanger unit--the gas inlet of which, exhaust gas from the kiln is directed--has means for discharging the preheated material into the first mentioned heat exchanger unit at or near its inlet end.
German Publication No. 2,262,213 to Rohrbach (laid open for inspection Dec. 12, 1974) relates to a method of heat treating such raw materials by directing preheated raw materials from several strings of cyclone-type suspension preheaters to a single calcinator for clacining material prior to feeding it into a kiln. Waste gases from a clinker cooler are directed to the preheaters to utilize the heat therefrom in preheating the raw material.
Commonly assigned copending application Ser. No. 603,867, filed Aug. 11, 1975 relates to a method for heat treating such pulverous raw materials prior to subjecting them to further heat treatment in a kiln by dividing the gas flow and the raw meal flow into substantially equal divisional flows, with each flow being associated with a sub-stage of a final stage of a preheater. Other related patents--which were considered in the examination of my U.S. Pat. No. 3,864,075 --are as follows: U.S. Pat. No. 1,817,048 to Washburn; U.S. Pat. No. 3,037,757 to Deussner; U.S. Pat. No. 3,452,968 to Shimizu et al.; and U.S. Pat. No. 3,664,650 to Weber et al.
If the granular or pulverulent material such as cement raw meal is to be treated according to prior art methods, at least the greater part of the calcination process will take place almost exclusively near the air inlet end of the heat exchanger unit which is fed with hot waste cooling air. Also, the combustion of the fuel--nourished by the wate cooling waste required for performing the calcination process will take place in that locality. No worthwhile calcination will take place in the kiln nor in the heat exchanger unit which is fed with exhaust gas from the kiln.
With the earlier arrangements, the amount of hot waste cooling air drawn through the kiln per time unit is small and amounts only to that which contains enough oxygen to nourish the combustion of fuel at the burner in the kiln for carrying out the sintering. The remaining part of the hot waste cooling air will by-pass the kiln. and be led to the heat exchanger unit in which the calcination process is effected. This may result in the so-called pre-cooling zone located in the rotary kiln betweeen the mouth of its burner pipe and the clinker discharger outlet or outlets (as the case may be) not functioning properly, i.e. the clinker will not be precooled to the desired extent before leaving the kiln, because the amount of cooling air passing through the pre-cooling zone is too small, and the clinker cooler may be damaged.
Another drawback arising from the reduced amount of gas drawn through the kiln and originating from the hot waste cooling air passed into the kiln is that the alkalis--which almost invariably will be released near the exhaust gas outlet end of a kiln for burning cement clinker--will be concentrated in a comparatively slight amount of combustion gas with the result that there will be a tendency to encrustration in the lower part of the riser pipe leading from the material inlet end of the kiln to the adjacent heat exchanger unit. I have developed a unique method and plant which avoids these drawbacks while providing a new and improved approach to heat treating such raw materials as cement raw meal.